The invention relates to a filter apparatus for fluids, in particular for thermoplastic synthetic plastics material fluid, with continuous cleaning of the surface of the filter, comprising a housing for a stationary, metallic, substantially planar filter to which the fluid to be filter is supplied through at least one upstream channel and from which the cleaned fluid is conducted off the housing through at least one downstream channel, at least one scraper provided with some scraping edges lying in a common plan and following curved lines, at least one drive means for rotation of the scraper so that the scraper strokes over the upstream surface of the filter and thereby strips off the impurities adhering thereon and carries them towards the center of the filter, wherefrom the impurities are conveyed off by at least one worm through at least one outlet channel extending from the center of the filter and leading out of the housing.
Filter apparatus for thermoplastic synthetic plastics fluid operating continuously in the most cases show as a filter a cylindric sleeve fixed in the housing (EP-A-411,163), which sleeve on its outside is surrounded by the scraper in form of a spiral band. This, inter alia, has the disadvantage the problems arise during assembly or disassembly, for example for service purposes, due to the length of the filter cylinder, in particular for bigger units. The filter sleeve consists of thin sheet metal and, therefore, is comparatively weak. When axially assembling or disassembling such a filter sleeve, deformations cannot be avoided, in particular due to the scraper that frictionally engages the filter sleeve. In order to ensure the necessary penetrability of the filter openings, these openings must show a greater diameter on the outside of the filter shell that at the inner side. Economically, such openings can only be produced by lasers, and this has a consequence that these bores are produced at the planar filter element which after production of the bores is rolled round and is welded at the location of the joint. This makes it difficult to obtain the necessary exactness. It is not possible to product the filter openings after rolling round the sleeve because the laser operators cannot be introduced into the sleeve. A further disadvantage consists in a lack of operation reliability. This results from that the spiral band, in dependency of its direction of rotation, tends either to jam at the filter sleeve or to surround it only loosely. In the first case there is at least the disadvantage of an increased power requirement in order to overcome the arising friction forces, and what is more, there is a breakdown of operation or a tearing of the filter element. In the second case, the scraper does no more sufficiently engage the filter and, therefore, cannot more reliably convey off the impurities that collect at the outside of the filter.
For this reason, it has been already proposed to provide a continuously operating filter apparatus with a planar sieve (DE-A-3,335,949, DE-A 2,324,581) and to construct it in the initially described or in a similar manner. However, it is of disadvantage within this construction, that it is suitable only for relative small throughputs. The reason for this is that the planar sieve plate used within this know construction tends to deformations under the pressure of the supplied fluid containing impurities, which pressure may amount to about 350 bar, so that the scraping edges of the scraper do not more exactly contact the surface of the sieve plate to be cleaned. Therefore, there is soon a soiling of the sieve plates and therefore a breakdown of operation. Similar disadvantages occur within further know apparatus in which stripper elements are disposed a small distance apart from the filter (GB-A 640,276) or in which instead of scrapers impellers are provided (U.S. Pat. No. 4,657,636). Restrained textile filters (FR-A 1,356,496) cannot be used for high pressures.
The invention has on its task, on the one hand to substantially increase the throughput in comparison to the know construction, and on the other hand to provide for a reliable cleaning of the filter. The invention solves this task by the features that the filter is constituted by at least two filter disks disposed in parallel to each other and spaced apart from each other, that the fluid is supplied through the upstream channel at the outer periphery of the filter disks, that the scraper acting upon both filter disks is disposed between the filter disks, each of the scraping edges of the scraper being formed by some scraper elements spaced from each other but disposed along the lines and being pressed onto the filter disks, and that collection spaces for the cleaned fluid are provided at the opposing sides of the filter disks, which collection spaces are connected with the downstream channel in the region of the outer periphery of the filter disks. The provision of two filter disks doubles the active filter surface without increasing the effort for upstream channels and downstream channels. To divide the scraping edges of the scraper into some scraper elements per line enables on to individually adapt these scraper elements to the deformations of the filter occurring during operation so that, in comparison to the lastly described known apparatus, cleaning of the filter is substantially improved and thereby the tool life is increased at a substantially increased throughput. The increased safe of cleaning of the two filter disks by the single scraper elements enables one also to substantially increase the active area of the filter disks in comparison to the lastly described known construction even if this increase has as a consequence that the filter disks are deformed under the pressure of the material to be filtered.
In comparison to the first described known construction there is the advantage that the planar filter disks can be produced in a substantially easier manner and thereby showing a greater exactness than a filter sleeve, because there is no rolling round. Thereby, also the assembly and disassembly of the filter into or out of the housing is facilitated, as well as mounting and demounting during service operation. Since there is no more a jamming of the scraper on the filter, also the reliability of operation is increased.
The supply of the material to be filtered from the outer periphery of the filter disks between them enables one also to dispose the upstream channel or the upstream channels radially with respect to the axis of the filter, and this also for the downstream channel for the filtered material or, respectively, for the downstream channels for same. Thereby, one is more independent in the disposition of the supply of the material to be filtered, when compared with the lastly described known construction, at which the supply must be made in axial direction of the filter so that the drive means for rotation of a damming member carrying the scraping edges must be disposed at that side of the apparatus that is opposite to the supply side of the material.
Filter comprising two parallely disposed filter disks to which the fluid to be cleaned is supplied through a common supply channel, are known for constructions which can be backwashed and which work without scrapers (U.S. Pat. No. 3,361,263).
According to a preferred embodiment of the invention, each scraper is connected for rotation with a shaft disposed coaxially to a worm carrying off the impurities and connected to a drive means being independent from the drive means for the worm. In such a manner, the scraper elements can be rotated with a speed being different from that of the worm carrying off the impurities so that a better adjustment to the present circumstances can be obtained.
By the feature that within the inventive apparatus both axial directions of the filter apparatus are free for the drive means, there is the possibility within the spirit of the invention for variants with respect to disposal and number of the worms carrying off the impurities. Thus, it is possible within the spirit of the invention, to provide two coaxial worms in two downstream channels which worms carry off the impurities in two opposite directions. However, also all impurities can be carried off towards one side, either by one single worm only or within the spirit of the invention by two worms disposed coaxially within each other in one common downstream channel and conveying the impurities in the same direction towards a common outlet opening of the downstream channel.
For the construction of the single scraper elements, there are two particularly advantages variants within the spirit of the invention: On the one hand, according to the invention the scraper elements may be formed by sheet metal angle pieces that with always one leg are fixed to a common scraper carrier member and with the other leg constituting the scraper edge resiliently engaging the filter disk. On the other hand, a particular favourable embodiment of the invention consists in that each scraper element is fixed by means of at least two pins to a scraper carrier member, the pins engaging associated bores with play, each scraper element being provided on its side facing the filter disk with a protruding scraping ledge. These pins may either be clamped within the scraper element or within the scraper carrier member, the associated bores being disposed in the respective other member. In each case, the scraper elements cannot slide off the pins because they engage the filter disks. To the contrary, the scraper elements are automatically pressed against the filter disks by the pressure of the material to be filtered without that therefor separate springs are necessary. The protruding scraper ledge, namely, has as a consequence that the remaining surface of the scraper element facing the filter disk is smaller than the opposing surface of the scraper element. The high pressure of the supplied medium to be filtered, therefore, presses each scraper element towards the neighbouring filter disk, as long as the pressure of the material to be filtered is retained. Within this, it is of advantage that these circumstances remain substantially unchanged over the life of the respective scraper element. Even if namely the scraper ledge wears by and by, the scraper element is still pressed by the pressure of the material to be filtered towards the filter disk as long scraper ledge sufficiently protrudes from the neighbouring surface portion of the scraper element.
As already mentioned, the requirement to reliably convey the impurities scraped off the filter disks and conveyed towards their center, is the more important, the greater the throughput of the filter apparatus is. Mostly the impurities constitute a comparatively compact mass which sometimes contains also bigger foreign bodies, for example wire pieces, that may clamp between the filter disk and the scraper element and, therefore, may impede the rotation of the scraper. Furthermore, considerable shearing forces appear at greater throughputs or, respectively, filter disks of great area, which forces must be overcome when rotating the scraper, what requires a high power for the drive, that, what is more, is frequently not constant, but shows uncontrollable peaks in dependency from the kind, size and frequency of the impurities. Furthermore, it happens again and again, that the plasticizing device preceding the filter apparatus delivers portions of the material that are only partially plasticized, what also causes increased shearing forces. In order to overcome also these difficulties, according to a further embodiment of the invention the construction is so chosen that a scraper carrier member carrying the scraper elements is provided with recesses extending from its periphery and, respectively, or with openings disposed between the scraper elements, the edges of these recesses or openings being at least in sections provided with cutters acting on the supplied material. On the one hand, these recesses or, respectively, openings constitute free spaces of the scraper carrier member, so that the shearing forces acting on the synthetic plastics material or other material to be filtered, are decreased. On the other hand, the cutters disposed at the edges of these recesses or, respectively, openings, at the same time comminute bigger impurities, and this already immediately after the moment at which these impurities are supplied from the upstream channel into the periphery region of the respective filter disk. The more these impurities are comminuted, the easier the scraper carrier member can be rotated and the easier the comminuted impurities can be conveyed towards the center of the respective filter disk, where they reach the downstream channel and are conveyed off. This substantially contributes to a lengthening of the intervals of the filter exchange, in particular for mineral or metallic impurities, because bigger, hard, sharp-edged impurities may scratch the filter surface when being conveyed along the area of the filter disk towards the output worm. This danger is the less, the finer these bigger impurities are comminuted, because comparatively small particles of the impurities can easier accommodate between neighbouring scraper elements than bigger particles.